In recent years, regulation for exhaust gas has become stricter, and lean burn engines, direct injection engines, etc. have come into wide use. In this connection, NOx-occluding catalysts capable of effectively purifying the NOx present in an exhaust gas, in a lean atmosphere have been put into practical use. As the NOx-occluding component used in the NOx-occluding catalyst, there are known alkali metals such as K, Na, Li, Cs and the like; alkaline earth metals such as Ba, Ca and the like; rare earth elements such as La, Y and the like; and so forth. Recently, attention has been paid to the addition effect of, in particular, K which is superior in NOx occludability at high temperatures.
NOx-occluding catalysts are generally constituted by loading a catalyst layer containing the above-mentioned NOx-occluding component, on a carrier composed of an oxide type ceramic material (e.g. cordierite) or a metallic material (e.g. a Fe-Cr-Al alloy). Such a carrier, however, has a problem; that is, it is easily corroded and deteriorated by alkali metals or some alkaline earth metals, particularly, K, Na, Li and Ca when they become active under the high temperatures of exhaust gas. In particular, a cordierite carrier constituted by an oxide type ceramic material has a serious problem; for example, the carrier reacts with the above-mentioned alkali metal, etc. and generates cracks.
To prevent such carrier deterioration, JP-A-2000-279810 discloses a technique which comprises allowing the porous oxide particles constituting a catalyst layer (a catalyst-loading layer) to contain silicon which is reactive with alkali metals, allowing the alkali metal present in the catalyst layer near its boundary with a carrier (a substrate) to react with the silicon before the alkali metal migrates into the carrier, thereby preventing the migration of the alkali metal into the carrier. The above literature discloses also a technique which comprises forming a zirconia layer between a carrier and a catalyst layer to allow the zirconia layer to prevent the migration of the alkali metal present in the catalyst layer, into the carrier.
However, of the techniques disclosed in the JP-A-2000-279810, the technique of allowing the porous oxide particles to contain silicon had a demerit in that while the migration of the alkali metal into the carrier is prevented, the alkali metal reacts with the silicon and loses its NOx adsorbability. In the case of forming a zirconia layer as an anti-corrosive between the carrier and the catalyst layer, the denseness of the zircona layer as an intermediate layer is important to prevent the migration of the alkali metal effectively; however, it is practically very difficult to form the zirconia layer on the porous carrier without generation of any crack or pinhole, remaining of any exposed carrier portion, or the like.
The present invention has been made in view of the above situation, and aims at providing a catalyst body such as NOx-occluding catalyst, which comprises a carrier and a catalyst layer containing an alkali metal and an alkaline earth metal, loaded on the carrier, wherein the deterioration of the carrier caused by the alkali metal, etc. is prevented effectively and the long-term use of the catalyst has been made possible.